The present invention is directed generally toward coal preparation plants and, more particularly, toward a system and method for monitoring and controlling the feed and operating parameters of water-only cyclones to improve cyclone performance.
Coal preparation plants separate organic and non-organic solid particles by their specific gravities. A feed of raw mined coal is input to the coal preparation plant, which separates the raw mined coal into clean coal and refuse. Coal preparation plants typically utilize two basic processing methods for separating coal from rock and varying proportions of striated rock in the coal from the higher quality coal. These two processing methods include heavy media and water based separation methods. Heavy media separation is the most common separation process for larger sized particles, whereas water based separation processes are more commonly utilized for the finer sized particles. Water based separation methods depend exclusively on increased gravitational forces, slurry velocity and cyclone geometry. Water-only cyclones and spirals are typically utilized in coal preparation plants for separating the finer sized raw coal particles. Ultrafines are conventionally either cleaned in froth flotation or discarded.
Water-only cyclones include a cylinder with a truncated cone bottom. Gravitational forces are developed in the cyclone from the pressurized feed slurry fed near tangentially into the feed inlet with discharges at the top and bottom of the cyclone. The truncated cone allows a refuse bed to form. A relatively long vortex finder siphons the lighter clean coal particles. Water-only cyclones include various process inefficiencies, and therefore are not typically utilized as a single stage device in coal preparation plants for processing the finer sized raw coal particles. These process inefficiencies result in either clean coal being erroneously misplaced as refuse, or refuse being erroneously misplaced as clean coal. The water-only cyclone inefficiencies are typically associated with changes in the tonnage of feed solids reporting to the water-cyclone as a result of changes in the percent feed solids by weight and/or feed solid washabilities (e.g., the percentage of either high specific gravity rock and/or near gravity middlings in the raw coal feed), commonly referred to as head ash.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
A system is provided for controlling the operation and performance of water-only cyclones used in coal preparation plants for separating a fine raw coal slurry into fine clean coal and fine refuse slurries. The inventive system includes a variable speed pump feeding the fine raw coal slurry from either a sump or the underpan of a deslime screen to a water-only cyclone. A level sensor measures the level of the fine raw coal slurry in the sump or underpan and generates a signal representative of the measured level value. A pressure sensor measures the pressure of the fine raw coal slurry at the water-only cyclone inlet and generates a signal representative of the measured pressure value. A speed control device is also provided which receives the level and pressure sensor signals, and controls the speed of the variable speed pump in response thereto. For instance, if the level of the fine raw coal slurry in the underpan rises too high, the speed control device is responsive to the level sensor signal to increase the speed of the variable speed pump to lower the slurry level. Conversely, if the level of the fine raw coal slurry in the underpan falls too low, the speed control device is responsive to the level sensor signal to decreases the speed of the variable speed pump to raise the slurry level.
Adjustments in pump speed will also directly affect the pressure at the inlet of the water-only cyclones. The pump speed will be adjusted to maintain the pressure within a defined range limit. If maintaining underpan levels exceeds the maximum pressure limit, the plant feed tonnage will be manually reduced.
The inventive system also includes a nuclear density gauge measuring the specific gravity of the fine raw coal slurry pumped to the water-only cyclone. The nuclear density gauge generates a signal relative to the measured specific gravity of the slurry. A makeup water control valve receives the signal from the nuclear density gauge control system to adjust the addition of water to the fine coal slurry maintaining a set point specific gravity of the fine raw coal slurry. The water may be introduced into the fine raw coal slurry by either adding the water to the raw coal feed at the feedbox, along the deslime screen, or inputting the water to the fine raw coal slurry at a location proximate a suction of the variable speed pump.
If more than one water-only cyclone is utilized by the coal preparation plant, a distributor is provided which receives the fine raw coal slurry from the variable speed pump and separates the fine raw coal slurry into an appropriate number of feed slurry portions. The pressure sensor is provided at the distributor to measure the pressure of the appropriate number of feed slurry portions, and the signal generated by the pressure sensor is reflective of the pressure value.
Using the inventive assembly, the feed pressure to the water-only cyclone is monitored, and the specific gravity of the slurry is controlled. Also, the level of the slurry in the sump or underpan is controlled so that the level does not overflow as a result of controlling the feed pressure and/or the specific gravity.
A method according to the present invention is also provided for controlling the performance of water-only cyclones. The inventive method includes the steps of measuring the level of the fine raw coal slurry in the sump or underpan, measuring the pressure of the fine raw coal slurry pumped to the water-only cyclone, and controlling the speed of the variable speed pump in response to the measured level and pressure values.
The inventive method further includes the step of maintaining a select specific gravity of the fine raw coal slurry pumped to the water-only cyclone. The select specific gravity is maintained by adding water to the fine raw coal slurry to dilute the slurry in response to the measured specific gravity value. The water may be introduced into the fine raw coal slurry by either adding the water to the raw coal feed as the raw coal feed is fed to or along the deslime screen, or inputting the water to the fine raw coal slurry at a location proximate a suction head of the variable speed pump.
It is an object of the present invention to:
improve water-only cyclone operation and performance; and
control the feed and operating parameters of a water-only cyclone to improve cyclone performance.
Other objects, aspects and advantages of the present invention can be obtained from a study of the specification, the drawings, and the appended claims.